In a conventional pilot valve apparatus shown in FIG. 9, a valve body 1 has four slidable spools 4 to connect an oil port 2 to outlet ports 3 and to disconnect the oil port 2 from the outlet ports 3. A guide cylinder 5 is provided coaxially with respect to each of the spools 4, and a retainer 6 is disposed below the guide cylinder 5. The retainer 6 is pressed against the guide cylinder 5 by a main spring 7 do as to retain a piston 8 inserted into the guide cylinder 5 in such a manner as to be slidable therealong at an upper position. A spring 9 retains the spool 4 at a disconnecting position. A plate 10 is mounted on an upper portion of the valve body 1. The plate 10 has a hole 11 into which a proximal end 12a of a universal joint 12 is threadedly engaged with an upper portion of the valve body. A lever mounting shaft 13 is provided on a distal end 12b of the universal joint 12. Also, a disk 14 is mounted on the universal joint 12. When a lever 15, provided on the lever mounting shaft 13, is pivoted back and forth or to the right and left, one of the pistons 8 is pressed down to shift the associated spool 4 to the connecting position and thereby supply the oil in the oil port 2 to the outlet port 3 via an oil hole 4a of the respective spool 4.
Such a pilot valve apparatus is employed to change over a pilot pressure operated type operating valve for supplying an oil to an actuator in a work vehicle, such as a power shovel. A change-over of the operating valve is achieved when the oil is supplied to one of a pair of pilot pressure chambers of the pilot pressure operated type operating valve respectively connected to a pair of outlet-ports 3 by the pivoting of the lever 15. When the lever 15 is pivoted, for example, in the forward direction, the oil is supplied from the outlet port 3 located on that side. When the lever 15 is pivoted in the rearward direction, the oil is supplied from the outlet port 3 located on that side. Thus, the direction of the change-over of the operating valve is determined by the direction of the pivoting of the lever 15 to determine the direction of the operation of the actuator.
In some work vehicles, the direction of the operation of the actuator is made opposite from the direction of the pivoting of the lever 15. In that case, the direction of the flow of the oil outputted from the two outlet ports 3 must be changed before the oil is supplied to the pair of pilot pressure chambers of the operating valve.
Hence, as shown in FIG. 9, a solenoid operated directional control valve 17 is provided between a pair of conduits 16 respectively connected to the pair of outlet ports 3. The solenoid operated directional control valve 17 is operated by a limit switch 18 operated by the pivoting of the lever 15 to reverse the flow of the oil in the pair of conduits 16.
That is, the solenoid operated directional control valve 17 is retained at a first position which ensures that the oil flows in a forward direction, as shown in FIG. 10. When a solenoid 17a is energized, the solenoid operated directional control valve 17 is changed over to a second position. The solenoid 17a is energized by the limit switch 18.
In such a pilot valve, although the oil output from the outlet port 3 is reversed and then supplied to the pilot pressure chamber of the operating valve, the provision of the solenoid operated directional control valve, the limit switch, the wiring and conduits is required, thus increasing production cost and reducing reliability.
FIG. 11 is a graph showing the relationship between the output pressure of the conventional pilot valve apparatus and the lever stroke. As shown in FIG. 11, when the lever is shifted to stroke S.sub.1, the pilot valve apparatus produces the output pressure P.sub.1. Afterwards, the output pressure of the pilot valve apparatus increases in proportion to the lever stroke until it is equal to inlet pressure P.sub.2 at stroke S.sub.2.
In a vehicle provided with actuators controlled by the pilot valve apparatus, there are requirements of control of an actuator, for example, an engine or brake valve, on the basis of the results of detection of the controlled state of another actuator.
Such examples include control of a governor of the engine in such a manner that the engine speed is increased from the idling state to a rated speed by the detection signal, control of a solenoid operated valve such that a parking brake for a swing motor or a travel motor is switched off, and control of an oil to the brake valve. To achieve these objectives, a pressure switch is conventionally provided at the outlet port 3 to detect the operation of the lever 15. Alternatively, pivoting of the lever 15 is detected as a control initiating signal for another actuator by detecting the pressure of the limit switch or by an ON signal thereof.
However, as shown in FIG. 11, when the operation stroke is S.sub.1, the output pressure is P.sub.1, which is, for example, 6 kg/cm.sup.2. Since the detection signal must be outputted before the hydraulic actuator enters the actuator control area, it should be outputted by the time the output pressure is P.sub.1. Furthermore, the pressure switch is designed to output no detection signal when the output pressure is between zero and P.sub.3, which may be 3 kg/cm.sup.2, to prevent erroneous operation.
Thus, the activated area of the pressure switch is in the narrow region between P.sub.3 and P.sub.1, which may be between 3 and 6 kg/cm.sup.2. Consequently, a high-precision pressure switch must be provided to accurately output the detection signal. This requires a very expensive pressure switch.
In the case of the limit switch, a detection signal is outputted when the operation stroke of the lever 15 is between zero and S.sub.1. Therefore, a highly accurate positioning of the limit switch is necessary, making the mounting work thereof troublesome.